Strange 23andMe matching result: 2 males "completely identical" on X chromosome

+5 votes
1.4k views
Perusing DNA data from 23andMe for an adoptee who is related to me in a fashion as yet undetermined.  He has a new match with another man who also matches to me. The adoptee's matched to this man include matches on 3 segments of  the X chromosome. One of those three matches is a "half" identical match, which is what 23andMe reports when two people match on one copy of a particular chromosome. The other two matches are "complete,' which is supposed to mean that the people match on both copies of the chromosome (for example, my sibling and I have a bunch of "complete" matches in addition to a bunch of "half" matches; the complete matches are places where we both inherited the same chromosome material from both of our parents). They have no other "complete" matches.

Since men have only one copy of the X chromosome, I can't imagine how two men (regardless of their relationship) could possibly have any "complete" matches on the X chromosome. Any insights out there?
in The Tree House by Ellen Smith G2G Astronaut (1.2m points)
Imagine the family trees of these two men tracing back to their most recent common ancestor. It will not contain any male-male links.

3 Answers

+6 votes
Hi I haven’t done any genealogy DNA but I am a geneticist.  Men only have one X as you said inherited from their mother.  The mother has two X’s (since she is female) to potentially give.  If the men were brothers from the same mom they could each have inherited the same X from the mother.
by Kyla H G2G6 Mach 4 (46.3k points)
It is not biologically possible for a man to have two copies of an X chromosome. I'm wondering if anyone  has insight into what 23andMe is reporting here.
As an aside - and really less to go with genealogy but more biology - it's not only possible for a man to have two copies of an X chromosome (or three or even more) and it happens quite often - roughly one in every thousand male births.  

It's called Klinefelter Syndrome and in many cases it leads to infertility but it is not unknown for some to father children in some cases.  Recently IVF treatment has lead to a huge increase in the odds of doing so.    By the sheer weight of probability, some people here will have a male with Klinefelter in their tree.
+2 votes
I'm more puzzled by what sort of test result they would call a half-match.
by Anonymous Horace G2G6 Pilot (568k points)

laugh Their term is "half identical." In situations where the two people have two copies of a chromosome, if two testers have matching genes on both copies of the chromosome, the call the matching segments "completely identical." For chromosomes that come in pairs, "half identical" means that the test found a patch where both testers had the same sequence of genes over a meaningfully long segment of the chromosome on half of their genes, but no match for the other half of the genes on the same segment. 

Logically, two men's matching segments on the X chromosome could be called "completely identical", since there's no second set of nonmatching  DNA. But 23andMe usually calls X chromosome matches between two men "half identical." Why would they change their terminology to call some such matches "completely identical"?

I have a half identical match on the complete  x  and all other chromosomes  with my father ; the other half of my DNA  came from my mother.

I assume  that because a man only has 1 x chromosome any match has to be a complete match (on that segment) . Edit I didn't see Ellen's reply about 23 and me changing the terms used

(Wonder what happens if the match is on the pseudoautosomal bits on the sex chromosomes)

+5 votes
I just looked at my two sons' 23andMe results. They match three segments on the X chromosome which is listed as completely identical. Since they each only have one copy of X, they are completely identical. I, as a female, am listed on my son's report as sharing the whole X chomosome as a half identical match. I think 23andMe is just using the terminology a bit differently than some other sites.
by

So 23andMe is listing all of your sons' X chromosome matches as completely identical? That makes logical sense.

The particular pair of men I describe are the first instance where I've seen 23andMe use "completely identical" for two men's X chromosome matches. It's good to know that this is not unique to this pair of men. What particularly perplexes me is the indication that two men have some matching segments on the X chromosome that are half-identical, but others that are completely identical.

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